Fuel gauge not working

[kevinw]

About four month ago I noticed my left (panel) fuel gauge was reading about half full. I knew this to be incorrect because I had just refueled. I lightly hit the top of the panel with my fist a few times and it corrected itself and showed full. Up until now it has worked normally. A few days ago I refueled the plane and as I was running back to my hanger I noticed the same problem; full fuel but a reading of just over half in the left tank. I tried the same corrective measure as before with no luck. Before I run it into the shop can someone tell me if this is a common problem and also if there's something i can do to fix it. It's in a 1982 M20J. Thanks guys!

[ArtVandelay]

About four month ago I noticed my left (panel) fuel gauge was reading about half full. I knew this to be incorrect because I had just refueled. I lightly hit the top of the panel with my fist a few times and it corrected itself and showed full. Up until now it has worked normally. A few days ago I refueled the plane and as I was running back to my hanger I noticed the same problem; full fuel but a reading of just over half in the left tank. I tried the same corrective measure as before with no luck. Before I run it into the shop can someone tell me if this is a common problem and also if there's something i can do to fix it. It's in a 1982 M20J. Thanks guys!

I have the same problem, I plan on having the connections cleaned and re-tighten when I have the glare shield off for some other work. Let me know if you find a solution.

[N201MKTurbo]

You should determine if it is a mechanical or electrical problem. 

 

Either the signal to the gauge is wrong or the gauge movement is sticking.

 

Next time it is showing 1/2 full put a volt meter on the terminals on the gauges and compare the right and left. if they read the same then the gauge is sticking. If not then there is an electrical problem. If there is an electrical problem, the first thing I would do is take the plastic circular connector apart that connects the gauge cluster to the plane and clean it and re-seat it.

[philipneeper]

I know on my C, i just recently had my senders overhauled. I used Lockhaven in PA. You can test your senders yourself before you send them off. What i would do is test the senders first then you know its the gauge. 

[N601RX]

For the planes with 2 senders per tank, one inboard and one outboard they are wired in series. Each sender is 30 ohms for a total 60. If your reading 1/2 full when you know your full it's likely there is a problem with one of the senders.

[kevinw]

For the planes with 2 senders per tank, one inboard and one outboard they are wired in series. Each sender is 30 ohms for a total 60. If your reading 1/2 full when you know your full it's likely there is a problem with one of the senders.

Yes, both times it sits at half full. You might be on to something. Thanks.

[N601RX]

I looked back at some notes I made when I put my jpi 900 in and something close to 0 ohms represents empty and something close to 60 represents full. One of the senders is grounded to the airframe and the other is floating. It sounds like the one that is floating may have had something happen and now be grounded on one side. This would give you 30 ohms and make it show half full. Another possibility is that one of them isn't floating and is stuck on the bottom of the tank.

[N201MKTurbo]

I would suspect the inboard sender grounding, but you said you fixed it by banging on the panel. That tells me that it is something in the panel.

[Piloto]

The problem is mostly related to the fuel sender float arm getting stuck. If the tanks were resealed it could be sealant residue on the float arm attachment point. I found that even on new Mooneys if there is no fuel for two weeks the arm may get stuck and you have to fist pound the bottom skin. Leaving the tanks at least half full helps on reducing the problem. 

 

José

[kevinw]

I would suspect the inboard sender grounding, but you said you fixed it by banging on the panel. That tells me that it is something in the panel.

I wonder if the banging was a coincidence. Where is the inboard sender located? I'd like to check if it's grounding. Thanks.

[kevinw]

The problem is mostly related to the fuel sender float arm getting stuck. If the tanks were resealed it could be sealant residue on the float arm attachment point. I found that even on new Mooneys if there is no fuel for two weeks the arm may get stuck and you have to fist pound the bottom skin. Leaving the tanks at least half full helps on reducing the problem.

José

tanks have not been repealed but the problem (this time) occurred after filling the tank. Worked fine, low on fuel, filled the tanks and noticed the problem taxiing to my hanger.

[N201MKTurbo]

The inboard sensor is located behind the interior upholstery just about where your butt is. You will have to remove the panels. Sometimes you can get the lower panels off without removing the window trim, but usually not.

The inboard sensor needs to be isolated from ground. It is mounted with insulators (same ones used on TO3 transistors) under the screws. You can check it with an ohm meter. The frame of the sensor should not be grounded. It should be between 0 and 30 ohms depending on how much fuel is in the tank. You can remove the wire from the outboard sensor which is much easier to get to. To be sure it is isolated. You could also remove the wire from the mounting screw on the inboard sensor but I wouldn't do that, it may start leaking.

[Awqward]

Does anyone have any photographs of these locations? The MM doesn't give a lot of guidance...I have an intermittent problem with my right indicator going to full which is a ground problem....

[N201MKTurbo]

The inboard sensor is behind the interior panels. The outboard sensor is in the cell just outboard of the fuel tank. Take off the first inspection panel that is not sealed. The fourth panel from the center of the plane.

 

As an airplane owner you are allowed to remove inspection panels and interior upholstery. At least in the US of A....

 

Also, you will get more guidance from the IPC then the MM.

[Awqward]

Thanks 1MK...yes my airplane is on the N-register (by active choice) and I have an FAA pilot certificate....so I am bound by and abide by the FARs :-)

I have previously accessed the outboard sender (during the annual) but not the inboard one....I will give it a go next weekend

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[carusoam]

Inboard one is not too challenging...

If you haven't seen it yet, be ready to purchase a gasket for the level sensor.

It is probably cork. It probably leaks.

It will make you feel better, because the aroma of 100LL in the cabin is unusually related to this.

Best regards,

-a-

[kevinw]

Took off in the plane and about two minutes into my flight I noticed the gauge was working again. Jose must have been right on this one; float arm stuck. In any case, It's always good to learn more how the system works and I now know what to look for if I have a sender problem down the road. Thanks everyone. -Kevin

[FlyingDude]

Hi guys,

Can you reach the floats from the fuel tank feed hole and manually move the floats to see if they are stuck?  

I had seen my uncle assist a cow give birth...  I can use similar gloves that come all the way to your shoulders...

Thanks.

[Jinxx500]

Good afternoon, I'm having the same issue with my 65 M20E.  The right side works fine but the left is stuck at approx 3/4 full, regardless of the actual full quantity. Any tips on accessing the float to see if that's the issue?

[carusoam]

1 hour ago, Jinxx500 said:

Good afternoon, I'm having the same issue with my 65 M20E.  The right side works fine but the left is stuck at approx 3/4 full, regardless of the actual full quantity. Any tips on accessing the float to see if that's the issue?

1) from the inside of the tank… open fuel cap, reach inside…

2) from the outside of the tank… remove interior sidewall unscrew four screws of the fuel level sensor…

Be extra smart… burning gasoline can be very painful…

PP thoughts only, not a mechanic…

best regards,

-a-

[affricate]

On 10/24/2014 at 6:59 PM, Piloto said:

The problem is mostly related to the fuel sender float arm getting stuck. If the tanks were resealed it could be sealant residue on the float arm attachment point. I found that even on new Mooneys if there is no fuel for two weeks the arm may get stuck and you have to fist pound the bottom skin. Leaving the tanks at least half full helps on reducing the problem. 

 

José

Where is the location of this area on the bottom skin?

[I have this friend..]

Let me save someone some time in the future...

1979 M20J Fuel gauges reading half full when tank is full, but only after being drained.

I know this is absolutely the inboard fuel sending units being stuck.

It is not an electrical problem.

Here are some clues.

1) It is repeatable to almost exactly half. There are two sending units per tank in series. Assuming conservative failure of open circuit a wiring issue would tend to make infinite resistance and read empty, maybe a poor connection could read a higher-than-normal resistance. However, reading exactly half or 16ish gallons on a 32 gallon tank and having that repeatable. It is very suspect that a poor connection would be just the right amount of resistance for this indication.

2) It happens specifically when the tank is run very low or drained.

3) In my case when I drained both tanks that indicated full, and then immediately filled back to top, they suddenly both indicated exactly half full.

4) The number one reason I KNOW this to be a stuck sending unit is because a $32 amazon borescope wired tied to a 3mm aluminum wire from amazon allowed me to go in from fuel tank fill hole, find the sending units and touch the sending unit arm. Eureaka, arm popped up, did to the other side, both tanks full.

Should I replace the sending units? well, here are some thoughts. :

1) Conservative fail. It won't cause you to run out of fuel, only make an unnecessary stop.

2) Likely to resolve itself on your first sharp bank.

3) It's repeatable but ONLY by draining my tanks to the very bottom. If you don't drain your tanks to below 4 gallons you will not likely see this, and you should not be flying with 4 gallons of fuel anyway. E.g. calibrating a Garmin G3X touch to work with OEM sending units (btw they have to be in voltage mode not resistance mode even though they work via resistance).

4) I experienced this many years ago on the same aircraft (drained to flush tank) and a borescope on aluminum wire worked then too. 15 yrs later draining the tanks I had to purchase a new scope.

Note1 the inboard sending units are on a rod connected to a small circular panel bolted to the vertical inboard edge of the tank e.g. accessed from inside. I know this from looking inside the tank with a borescope.

Note 2 not changing the sending units is not about the cost of the unit, it's about opening up a can of worms by breaking a seal. Also my cal curves look very nice, I don't believe this is a problem worth the effort to fix at the moment.

[Fly Boomer]

18 minutes ago, I have this friend.. said:

Should I replace the sending units?

Many of the Cies senders had to be bent.  I think Cies is pre-bending them now.

[redcatcher27]

On 5/14/2024 at 4:40 PM, I have this friend.. said:

Let me save someone some time in the future...

1979 M20J Fuel gauges reading half full when tank is full, but only after being drained.

I know this is absolutely the inboard fuel sending units being stuck.

It is not an electrical problem.

Here are some clues.

1) It is repeatable to almost exactly half. There are two sending units per tank in series. Assuming conservative failure of open circuit a wiring issue would tend to make infinite resistance and read empty, maybe a poor connection could read a higher-than-normal resistance. However, reading exactly half or 16ish gallons on a 32 gallon tank and having that repeatable. It is very suspect that a poor connection would be just the right amount of resistance for this indication.

2) It happens specifically when the tank is run very low or drained.

3) In my case when I drained both tanks that indicated full, and then immediately filled back to top, they suddenly both indicated exactly half full.

4) The number one reason I KNOW this to be a stuck sending unit is because a $32 amazon borescope wired tied to a 3mm aluminum wire from amazon allowed me to go in from fuel tank fill hole, find the sending units and touch the sending unit arm. Eureaka, arm popped up, did to the other side, both tanks full.

Should I replace the sending units? well, here are some thoughts. :

1) Conservative fail. It won't cause you to run out of fuel, only make an unnecessary stop.

2) Likely to resolve itself on your first sharp bank.

3) It's repeatable but ONLY by draining my tanks to the very bottom. If you don't drain your tanks to below 4 gallons you will not likely see this, and you should not be flying with 4 gallons of fuel anyway. E.g. calibrating a Garmin G3X touch to work with OEM sending units (btw they have to be in voltage mode not resistance mode even though they work via resistance).

4) I experienced this many years ago on the same aircraft (drained to flush tank) and a borescope on aluminum wire worked then too. 15 yrs later draining the tanks I had to purchase a new scope.

Note1 the inboard sending units are on a rod connected to a small circular panel bolted to the vertical inboard edge of the tank e.g. accessed from inside. I know this from looking inside the tank with a borescope.

Note 2 not changing the sending units is not about the cost of the unit, it's about opening up a can of worms by breaking a seal. Also my cal curves look very nice, I don't believe this is a problem worth the effort to fix at the moment.

"a $32 amazon borescope wired tied to a 3mm aluminum wire from amazon allowed me to go in from fuel tank fill hole, find the sending units and touch the sending unit arm. "

After having my M20F resealed with Edison at Wet Wingologist, I found my fuel senders on the left side stuck to the bottom. I expected it to correct itself after several landings, but it hasn't. I was able to easily pull the outboard sender off the bottom, but due to the baffling, unable to get to the inboard one. I purchased a 16 foot semi rigid snake borescope with led lights, along with 32 feet of 4 gauge aluminum wire.

Could you please fully describe the process you went through to get to your inboard sender? What did you use to tie the borescope to the wire?

Thanks in advance.

 

[Dammit Bill]

About four month ago I noticed my left (panel) fuel gauge was reading about half full. I knew this to be incorrect because I had just refueled. I lightly hit the top of the panel with my fist a few times and it corrected itself and showed full. Up until now it has worked normally. A few days ago I refueled the plane and as I was running back to my hanger I noticed the same problem; full fuel but a reading of just over half in the left tank. I tried the same corrective measure as before with no luck. Before I run it into the shop can someone tell me if this is a common problem and also if there's something i can do to fix it. It's in a 1982 M20J. Thanks guys!

You need to hit the bottom of the wing. Outboard, maybe 1 bay outboard and aft of the filler. It’s an access panel. You will hear it bouncing around. Try it w half tanks


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